1. Field of Invention
This invention relates to soft and flexible electrical heaters, and particularly to heating elements, which have soft and strong metal or carbon containing electrically conductive threads/fibers.
2. Description of the Prior Art
Heating elements have extremely wide applications in household items, construction, industrial processes, etc. Their physical characteristics, such as thickness, shape, size, strength, flexibility and other characteristics affect their usability in various applications.
Numerous types of thin and flexible heating elements have been proposed. For example, U.S. Pat. No. 4,764,665 to Orban et al. discloses an electrically heated fabric for use in gloves, airfoils and aircraft parts. In this patent the fabric is metallized after being formed in a glove structure, following weaving or arranging in a non-woven format. Copper bus bars are utilized for introduction of electrical current to the metallized textile. Having been made of a solid piece of fabric with metallized coating, this heating element doesn""t allow for flexibility in selection of desired power density.
The metallizing of the formed heating element results in a loss of significant economies of scale, only a small number of embodiments can be achieved, thus severely limiting the potential application of this invention. The ""665 design is also not conducive to tight hermetic sealing through the heater areas (no gaps inside), which can cause a short circuit through puncture and admission of liquid into the body of heating element. this element can""t be used with higher temperatures due to the damage caused to the polyaramid, polyester or cotton metallized fabric, described in the invention.
Another prior art example is U.S. Pat. No. 4,713,531 to Fennekels et al. Fennekels et al. discloses a sheet textile structure having resistance elements combined with it. These resistance elements comprise metallic fibers or filaments with a denier like that of natural or synthetic textile fibers, and with overall cross sectional thickness of 8 to 24 microns. The ""531 design suffers from the following drawbacks: being a sheet product, it is not conducive to hermetic sealing through the body of the heater (no gaps inside), only perimeter sealing is possible, which can result in a short circuit due to puncture and admission of liquid into the body of the heating element; yarns, comprising metal fibers, lack consistency of electrical resistance per given length, and their stretching, compression, or both, will result in very wide fluctuations in resistance, thus limiting the use of this technology in embodiments controlled by strict design and where an uncontrollable power output and temperature variability are unacceptable; yarns are very heavy: from 1 to 7 grams per 1 meter of yarn; the use of silver fibers makes these yarns very expensive; individual conductors have a large cross sectional thickness, each having an outer sheath of braided textile or elastomer.
Another prior art example is U.S. Pat. No. 4,538,054 to de la Bretoniere. The heating element of de la Bretoniere ""054 suffers from the following drawbacks: its manufacturing is complex requiring weaving of metal or carbon fibers into non-conductive fabric in a strictly controlled pattern; the use of the metal wire can result in breakage due to folding and crushing and it affects softness, weight and flexibility of the finished heater; it can not be manufactured in various shapes, only a rectangular shape is available; only perimeter sealing is possible (no gaps inside), which can result in a short circuit due to puncture and admission of a liquid into the body of the heating element; the method of interweaving of wires and fibers does not result in a strong heating element, the individual wires can easily shift adversely affecting the heater durability; the fabric base of the heating element is flammable and may ignite as a result of a short circuit; it is not suitable for high temperature applications due to destruction of the insulating weaving fibers at temperatures exceeding 120xc2x0 C.
A heating element proposed by Ohgushi (U.S. Pat. No. 4,983,814) is based on a proprietary electroconductive fibrous heating element produced by coating an electrically nonconductive core fiber with electroconductive polyurethane resin containing the carbonatious particles dispersed therein. Ohgushi""s manufacturing process appears to be complex, it utilizes solvents, cyanates and other toxic substances. The resulting heating element has a temperature limit of 100xc2x0 C. and results in a pliable but not soft heating element. In addition, polyurethane, used in Ohgushi""s invention, when heated to high temperature, will decompose, releasing very toxic substances, such as products of isocyanide. As a consequence, such heating element must be hermetically sealed in order to prevent human exposure to toxic offgassing. Ohgushi claims temperature self-limiting quality for his invention, however xe2x80x9cactivationxe2x80x9d of this feature results in the destruction of the heater. He proposes the use of the low melting point non-conductive polymer core for his conductive fabric-heating element, which should melt prior to melting of the conductive layer, which uses the polyurethane binder with the melting point of 100xc2x0 C. Thus, the heating element of Ohgushi""s invention operates as Thermal Cut Off (TCO) unit, having low temperature of self-destruction, which limits its application. U.S. Pat. No. 4,149,066 to Niibe et al. describes a sheet-like thin flexible heater made with an electro-conductive paint on a sheet of fabric. This method has the following disadvantages: the paint has a cracking potential as a result of sharp folding, crushing or punching; the element is hermetically sealed only around its perimeter, therefore lacking adequate wear and moisture resistance; such an element can""t be used with high temperatures due to destruction of the underlying fabric and thermal decomposition of the polymerized binder in the paint; the assembly has 7 layers resulting in loss of flexibility and lack of softness.
U.S. Pat. No. 5,861,610 to John Weiss describes the heating wire, which is formed with a first conductor for heat generation and a second conductor for sensing. The first conductor and a second conductor are wound as coaxial spirals with an insulation material electrically isolating two conductors. The two spirals are counter-wound with respect to one another to insure that the second turns cross, albeit on separate planes, several times per inch. The described construction results in a cable, which has to be insulated twice: first, over the heating cable and second, over the sensor cable. The double insulation makes the heating element very thick, stiff and heavy, which would be uncomfortable for users of soft and flexible products such as blankets and pads. The described cable construction cannot provide large heat radiating area per length of the heater as it would be possible with strip or sheet type of the heating element. The termination with electrical connectors is very complicated because of stripping of two layers of insulation. In addition, in the event of overheating of a very small surface area of the blanket or pad (for example several square inches), the sensor may fail to sense very low change in total electrical resistance of the long heating element. Such heating cable does not have Thermal-Cut-Off(TCO) capabilities in the event of malfunction of the controller.
The present invention seeks to alleviate the drawbacks of the prior art and describes the fabrication of a heating element comprising metal microfibers, metal wires, metal coated, carbon containing or carbon coated threads/fibers, which is economical to manufacture; does not pose environmental hazards; results in a soft, flexible, strong, thin, and light heating element core, suitable for even small and complex assemblies, such as handware. A significant advantage of the proposed invention is that it provides for fabrication of heating elements of various shapes and sizes, with predetermined electrical characteristics; allows for a durable heater, resistant to kinks and abrasion, and whose electro-physical properties are unaffected by application of pressure, sharp folding, small perforations, punctures and crushing. A preferred embodiment of the invention consists of utilizing of metal and carbon coated synthetic textile threads having a Thermal Cut Off (TCO) function to prevent overheating and/or fire hazard.
The first objective of the invention is to provide a significantly safe and reliable heating element which can function properly after it has been subjected to sharp folding, kinks, small perforations, punctures or crushing, thereby solving problems associated with conventional flexible heating metal wires. In order to achieve the first objective, the electric heating element of the present invention is comprised of insulated electrically conductive threads/fibers or metal wires. The conductive threads/fibers may be made of carbon, metal microfibers, textile threads coated with metal, carbon, conductive ink, or their combination. The proposed heating element may also comprise metal wires and their alloys. The conductive threads/fibers possess the following characteristics: (a) high strength; (b) high strength-to-weight ratio; (c) softness, (d) flexibility. The heating element core described in this invention is comprised of electrically conductive strips, sleeves, sheets or cables, which radiate a controlled heat over the entire heating core surface.
A second objective of the invention is to provide maximum flexibility and softness of the heating element. In order to achieve the second objective, the electric heating element of the invention contains thin (01 to 3.0 mm, but preferably within the range of 0.05-1.0 mm) individually insulated conductive threads/fibers or metal wires, which are woven or stranded into continuous or electrically connected strips, sleeves/pipes, cable or sheets. Another preferable configuration consists of extruding soft insulating material, such as, but not limited to PVC, polyurethane, temperature resistant rubber, cross-linked PVC or polyethylene around a multitude of conductive thread/fiber or wire electrical conductors provided that said electrical conductors are separated by said insulating material.
A third objective of the invention is to provide for the uniform distribution of heat, without overheating and hot spots, thereby solving the problem of overinsulation and energy efficiency. In order to achieve this objective, (a) conductive threads in the heating elements are separated by non-conductive fibers/yarns or insulating polymers, (b) one side of the heating element may include a metallic foil or a metallized material to provide uniform heat distribution and heat reflection. It is also preferable that the soft heating elements of the invention are made without thick cushioning insulation, which slows down the heat delivery to the surface of the heating unit.
A forth objective of the invention is to provide for ease in the variation of heating power density, thereby solving a problem of manufacturing various heating devices with different electric power density requirements. In order to achieve the forth objective, the electroconductive threads/yarns, fibers or metal wires are first insulated by polymer, creating multiple thin cables, which then laminated between woven or non-woven fabric, or interwoven with nonconductive threads into strips, sleeves/pipes or sheets with predetermined width, density of weaving and thickness. It is preferable that the strips and sleeves/pipes, sheets are made of combination of threads/fibers with different electrical resistance and/or include electrically nonconductive high strength polymer or inorganic (such as refractory ceramic or fiberglass) fibers.
A fifth objective of the invention is to provide a high level of temperature control. In order to achieve the fifth objective, at least one of the following is applied: (A) at least one thermostat is wrapped by the heating element or located in the place of multiple folding of the heating element core; (B) at least one of conductive threads or wires, running through the whole length of the heating element is connected to an electronic device operated by a change of electrical resistance, caused by the integral temperature change in the sensor, (C) a controller is set for periodic ON/OFF cycling.
The sixth objective of the invention is to provide a high level of safety, minimizing possibility of fire hazard. In order to achieve the sixth objective: (A) multiple thin conductive cables are reinforced by strong and flame retardant threads/fibers, and (B) the conductive media of the heating cables may comprise metal or carbon containing textile polymer threads/fibers having melting point from 120xc2x0 C. to 320xc2x0 C. The melting of the conductive threads/fibers results in breaking of electrical continuity in the heating system. Thus the proposed heating elements can operate as a high temperature fuse or TCO (Thermal-Cut-Off) device.
The present invention comprises a heating element containing soft, strong and light electrically conductive threads/fibers acting as conducting media. The heating element is also highly resistant to punctures, cuts, small perforations, sharp folding and crushing. It can be manufactured in various shapes and sizes, and it can be designed for a wide range of parameters, such as input voltage, desired temperature range, desired power density, type of current (AC and DC) and method of electrical connection (parallel and in series). A heating element preferably consists of non-conductive fibers/yarns or insulating polymers which are combined with electrically conductive individually insulated metal or carbon containing threads/fibers by weaving into or, laminating between layers of fabric, forming strips, sleeves/pipes or sheets.
The selected areas of the heating element core may contain electroconductive threads or wires to provide sensing of resistance/current change caused by variation of the heat. The heating element core is shaped by folding or assembling said individually insulated conductive media into a predetermined pattern. The terminals are attached to ends of said heating element core, which are electrically connected in parallel or in series. In the event of utilizing an alternating current, the individually insulated cables in the heating element core may be connected in such a way as to minimize electromagnetic field (EMF). The following are some of the methods for reducing/eliminating EMF in the preferred embodiments of the invention:
(a) Utilizing of the voltage step-down transformer;
(b) Utilizing of the voltage step-down transformer and rectifier;
(c) Utilizing of the AC rectifier with filtering capacitor;
(d) Providing of simultaneous opposite current flow in the individually insulated cables of the heating element core.